In RF transceivers today, the use of a reference clock or oscillator derived from a quartz crystal reference element is nearly ubiquitous. The performance advantages of this approach lie in the high frequency accuracy of piezoelectric quartz crystal resonators (on order of parts per million) and on the low-noise signal produced by the devices. For many applications, the cost and size of reference frequency generation hardware for implementations of this approach make up only a small fraction of the total cost of the communications equipment.
However, for other devices including, but not limited to, a new class of receivers, transmitters and transceivers which are being developed for short-range, low bit-rate applications such as wireless sensing, logistics and game controls, the cost of a crystal reference can represent a substantial percentage (e.g., 10% to 30%) of the total cost of the transceiver. Furthermore, while the cost of the integrated portions of the transceiver are expected to decrease substantially over time, the crystal cost is unlikely to decrease at the same rapid rate. Thus, the cost of the crystal as a percentage of the total cost of the transceiver may actually tend to increase over time.
Crystal reference elements do not currently lend themselves to integration on a silicon substrate with other circuit elements. This is because high quality factor (Q) resonators of the type used in reference elements are constructed from piezoelectric materials such as quartz that are not compatible with the silicon-based materials used in semiconductors. The crystal is, therefore, implemented as a discrete element outside of the integrated circuitry used to implement other elements of the device. The fact that the crystal is implemented as a discrete element has negative implications for both the cost and size of the transceiver.
Several communication techniques utilize circuitry that does not rely upon crystals for frequency stability as follows:
Inductive-Capacitive (LC) tuned receivers. While crystal reference circuits are common in modem communications equipment, equipment manufactured before 1980 sometimes utilized tuned LC (Inductor-Capacitor) circuits for frequency generation. Several examples of this are broadcast television receivers, broadcast radio receivers and short-wave radio receivers. Common elements in all of these systems are analog transmission format and high ratios of signal bandwidth to carrier frequency. Such applications are used only for analog formats.
Wideband Frequency-Shift Keying (FSK). While few commercial applications are in use, digital FSK systems with high modulation index exhibit tolerance to frequency offset. This class of system has support for digital modulation and can be made to support arbitrarily high ratios of carrier frequency to data rate. However, systems employing wideband FSK are inefficient in their spectral usage, since the occupied frequency band of the signal may be used by only a single user at a time. Furthermore, because the energy density of the wideband FM signal is not uniform across the frequency band, regulatory issues may arise with peak power density.
XOR-based processing of DSSS signals. In this approach, as described in U.S. Pat. No. 5,559,828, a DSSS (Direct Sequence Spread Spectrum) sequence is de-spread using an XOR (exclusive-OR) gate and a delay line. While this is effective in increasing tolerance to frequency offset, it does not produce coding gain and does not differentiate between codes, decoding all signals equally. Thus, several advantages of DSSS systems, including coding gain, code-division multiple access, and use of multiple codes in orthogonal modulation schemes, are lost using XOR processing.
None of these techniques is entirely suitable for use in certain digital communication systems such as, for example, those compatible with IEEE 802.15.4.